These radii coincide at \(C=|a|/M=0\)  and obey \(r_1 \le r_2\) . According to the direction of their rotation these two photon rings are divided onto prograde (in the same rotation direction as the black hole; for \(r_1\) ) and retrograde (in the opposite direction; for \(r_2\) ). Fixing \(C=1\)  (that corresponds to the case of a fast rotating BH) we find \(r_1=M\)  and \(r_2=4M\) . We attract the reader's attention to the fact of eqs. (REF ) and (REF ) describe stable photon orbits in the equatorial plane; in any other plane the location of photon orbits is not determined by (REF ), (REF ), and these orbits are generally unstable (see [1]}). We will use \(r_{1,2}\)  values as the reference points in our discussion of quantities in non-equatorial planes, in particular, in the (north) pole plane.
